The present invention relates to crimped bicomponent polyester fibers, formed of two different polyesters having different properties with respect to various mechanical and/or thermal treatments, especially treatments subsequent to shrinking or stretching operations. The fibers exhibit a hand similar to that of wool, and exhibit good processability in fiber systems.
Elastic crimped fibers of polymeric materials have been previously obtained by various mechanical processing methods, such as by assembling two filaments which are twisted in opposite direction (torsion twisting), and thereafter the twist is fixed and the filaments are untwisted, with the resulting filaments exhibiting a set crimp.
Bicomponent filaments have been obtained by extruding at least two different polymers, having different shrinking potentials, which may be of substantially the same or quite different nature, through the same spinneret die opening. These bicomponent fibers have a latent crimping ability which can be developed during subsequent processing steps. The results obtained by the prior art in the field of polyester filaments made of a single polymer, and crimped by conventional methods, such as the use of a heated sharp edge, of by false twisting, have never been satisfactory, as the crimp and the fiber elasticity disappear because of creep of the material. This problem is especially significant during those times when the filaments are subjected to tension in the manufacture of finished products, such as in the making of woven or knit fabrics, or in the process of manufacturing finished goods from such finished products. Polyester polymers are known to have significant plastic flow, so that the elasticity of the fibers, even when the fibers are composite fibers, rapidly decreases with fatigue.
The prior art has suggested that a crimped bicomponent fiber be made of a homopolyester and a copolyester made from diacids, at least one diacid being common to the two polymers, and a diol and one or more triols. For instance U.S. Pat. No. 3,454,460 discloses a composite polyester fiber made of polymethylene glycol terephthalate and a copolymer of polymethylene glycol terephthalate and of a terephthalate of another diol or of a polymethylene glycol isophthalate.
French Pat. No. 1,486,035 discloses a composite polyester filament made of polyethylene terephthalate and polyethylene terephthalate cross-linked with trimethylol propane.
French Pat. No. 1,442,768 discloses a composite fiber which is capable of developing a helical crimp and which is stable to heat and to deformation. Among components taught by this patent as suitable for making the composite fiber are polyethylene terephthalate and polybutylene terephthalate. The products of this patent have a slightly better crimp permanency than other prior bicomponent polyester fibers.
The prior art fibers described above, although they have elastic properties which are more permanent than those of textured fibers produced by conventional mechanical processes, are still not completely satisfactory for textile uses.
Copending, commonly assigned U.S. patent application, Ser. No. 456,476 discloses bicomponent polyester filaments wherein one filament component is polyethylene terephthalate and the other filament component is polybutylene terephthalate which has been sparingly cross-linked with trimethylol propane. These filaments exhibit improved permanency of crimping, compared to the other prior art acknowledged hereinabove.